Mechanical face seals are used on various types of machines and equipment, such as pumps, compressors and gear boxes, which have rotating shafts and a sealing chamber adjacent the shaft wherein a fluid in the sealing chamber is prevented from leaking along the shaft. Such mechanical seals include a pair of adjacent seal rings having opposing seal faces which define a sealing region therebetween. One of these seal rings typically is mounted on the shaft so as to rotate therewith while the other seal ring is non-rotatably mounted on a seal housing. The fluid being sealed is disposed on one edge of the sealing region, wherein the opposing seal faces at least reduce leakage of the sealed fluid across the sealing region.
Most liquid seals operate with the seal faces rotating in contact. However, due to asperities in the seal faces, some leakage may occur. In gas seals, the seal faces often are provided with grooves or recesses to generate hydrodynamic lifting forces. In this regard, the seal faces typically separate a small distance wherein a thin film of fluid forms between the seal faces to lubricate the seal faces and reduce wear therebetween. Additionally, the grooves or recesses may pump the fluid film toward the fluid being sealed to reduce leakage of the sealed fluid.
With respect to the specific constructions of mechanical seals, one representative mechanical seal is disclosed in U.S. Pat. No. 6,446,976 (Key et al), the disclosure of which is incorporated herein in its entirety by reference. In this mechanical seal, one of the seal faces includes a plurality of concentric shallow annular grooves which preferably are disposed near the seal ring diameter that is farthest away from the fluid being sealed. In general, the basic construction of mechanical seals and the use of relatively rotatable seal rings are well known, and a detailed discussion of such mechanical seals is not required herein.
More particularly, dry running lift-off face seals, also called fluid film, gap, or non-contacting face seals, have found application in both gas and liquid sealing applications in compressors and pumps. The fluid film between the seal faces allows the seal to operate with minimum heat generation and no wear.
Dry running lift-off face seals utilize a variety of shapes of shallow grooves to create lift between the seal faces, allowing the faces to run without contact. Existing examples include spiral grooves, radially tapered waves, and T-grooves. These various grooves are designed to provide a varying combination of hydrostatic and hydrodynamic load support to achieve separation of the seal faces by a small gap. Hydrostatic load support is created through the manipulation of the fluid pressures acting between the seal faces, and is not dependant on motion between the seal faces to create lift. Hydrodynamic load support is created through the active compression of the fluid between the seal faces due to movement of the fluid from a wide gap to a narrower gap, and requires relative motion between the seal faces to create lift. This relative motion typically occurs during shaft rotation.
The geometry of the shallow grooves determines the amount of hydrostatic and hydrodynamic load support created at a given set of operating parameters. The total load support provided must be in equilibrium with the pressure and mechanical forces that act to close the seal faces at a specified operating gap.
Also important to the design of lift-off face seals is the resistance of the fluid film to a changing gap, commonly referred to as film stiffness. This effect acts similar to a spring between the seal faces, increasing the forces for load support as the gap is narrowed, and decreasing the forces as the gap is made larger. This stiffness varies in a generally cubic relationship to the gap, and as a result encourages the seal faces to maintain equilibrium at a consistent gap. Changes to the geometry of the shallow grooves can have an effect on the stiffness value at a given gap, which then determines the stability of the seal at that gap. This is especially important during vibration or other off-design operation of the sealed equipment to maintain a consistent sealing gap and prevent damage to the seal faces from contact.
Lift-off shallow groove patterns for seal faces are typically designed to be either uni-directional or bidirectional depending on manufacturer's preference and application requirements. Uni-directional patterns such as spiral grooves generally produce higher film stiffness values than bidirectional patterns like waves or T-slots due to a stronger pumping effect. However, uni-directional patterns are only effective when the shaft rotates in one direction, but no longer operate during shaft rotation in the opposite direction. The disadvantage of uni-directional patterns is that they run with hard contact between the seal faces in events of reverse rotation, which often occurs in installations due to reversed flow of liquid through the pump. Bi-directional face patterns are effective in both directions of shaft rotation.
The objective of this invention is to provide an improved shallow groove pattern for bidirectional operation with a geometry that provides more efficient pumping and compression of the film gas, resulting in film stiffness values more closely resembling those of uni-directional patterns. This pattern also allows for the variation of hydrostatic and hydrodynamic effects based on the groove depths and pattern angles.
The invention relates to a reverse trapezoidal face pattern for mechanical face seals, as well as a mechanical seal or seal ring incorporating such feature. At least one of the seal faces includes a plurality of reverse trapezoidal features which are distributed in circumferentially spaced relation over the seal face. The term reverse trapezoidal feature or groove refers to a hydrodynamic face feature which has a generally trapezoidal shape defined by a short circumferential edge, a longer circumferential edge and side edges which extend radially between the circumferential edges. The reverse trapezoidal feature has the short circumferential edge located along a seal ring diameter so as to define an open edge which receives fluid into the reverse trapezoidal groove. This differs from a known trapezoidal face pattern which has the long circumferential edge located at the seal ring diameter which edge also receives fluid into the groove. The known trapezoidal face pattern creates significant differences in lift and fluid flow in comparison to the reverse trapezoidal face pattern of the invention. More particularly, the reverse trapezoidal face pattern serves to generate a hydrodynamic lift in either rotation direction which provides a stable separation of the seal faces that permits formation of a fluid film between the seal faces. Rather than the fluid film being generated solely by the static pressure of the fluid being sealed, the hydrodynamic lift features generate lift dynamically during shaft rotation to provide a lift load which is stable.
In the improved seal arrangement of the present invention, the groove pattern consists of a trapezoidal boundary defining the outer side edges and circumferential edges of each groove, and a sinusoidal or similarly shaped depth profile extending in the circumferential direction between the side edge boundaries. The trapezoidal boundary is not a true trapezoidal shape in appearance due to the circumferential stretching of the boundary along the annular seal face. The trapezoid shape is arranged with the narrower edge communicating with the high pressure side of the seal, and the wider edge forming a circumferential dam region towards the low pressure side of the seal.
The high pressure side of the seal can be either at the outside diameter or inside diameter of the seal face depending on the seal arrangement. The sinusoidal depth profile of the groove varies in depth amplitude radially, with the maximum depth amplitude occurring at the narrow edge of the trapezoid and the minimum depth amplitude occurring at the wider edge of the groove. The number of grooves and the circumferential width of the grooves can be varied to adjust the bias between hydrostatic and hydrodynamic lift.
The reverse trapezoidal shape of the groove pattern causes incoming gas from the high pressure side of the seal face to be drawn in and preferentially directed into the angled portion of the groove along the side edges, creating a pumping action. Since the wider circumferential edge is spaced radially from the short circumferential edge, the side edges angle away from the short circumferential edge and create a corner like area to which the incoming gas flows and then exits to the seal face. Due to the angle of the side edges, the reverse trapezoidal shape impedes recirculation of the fluid back to the high pressure side, which recirculation normally occurs in a conventional trapezoid face pattern. By reducing radial recirculation back to the high pressure side, the reverse trapezoidal face pattern generates improved lift and film stiffness while reducing fluid recirculation.
The sinusoidal shape is provided with a radially varying depth amplitude and acts to increase compression of the gas as it travels in the groove, while also allowing some recirculation effect at the high pressure boundary that helps to discourage possibly damaging debris and contamination from getting between the seal faces. The combined effect of the reverse trapezoid shape and sinusoidal profile is to increase the incoming flow of gas into the grooves and increase the compression on that gas, resulting in increased hydrodynamic lift and film stiffness. Modeling of the fluid dynamics in this pattern design has shown a significant increase in film stiffness at equivalent gap values with the reverse trapezoidal shaped pattern versus other bidirectional patterns such as a conventional trapezoid face pattern, spiral pattern, and a sinusoidal wave pattern.
In another aspect of the invention, the reverse trapezoidal face pattern may also be provided as the primary face pattern on a dry gas seal in combination with a secondary face pattern which results in an improved ability of the faces to operate with a controlled gap under normal pressure conditions as well as when the pressure direction across the seal face is reversed. This combined pattern employs the following features:
1) A primary shallow groove pattern along the periphery of the seal face towards the normal high pressure side of the seal face. This primary pattern is intended to create lift under normal operating conditions. This primary shallow groove pattern preferably comprises any of the sinusoidal or sine wave reverse trapezoidal face patterns of the present invention which define a dam region located radially between the outer the primary face pattern and a ring diameter spaced therefrom. Alternatively, the primary face pattern waves may be spiral grooves with a concentric dam, or even sinusoidal waves.
2) A secondary shallow groove pattern along the opposite diameter or periphery of the seal face towards the normal low pressure side of the seal face. This pattern is intended to create lift under reversed pressure conditions where the normal low pressure side reaches a higher pressure than the pressure defined on the opposite ring diameter. This secondary shallow groove pattern is preferably made up of sinusoidal waves.
With respect to the advantages of this alternate embodiment, one of the primary upset conditions that causes failure of lift-off face seals is a reversal of the pressure direction across the seal face. This upset can be caused either by a loss of the supply of barrier fluid pressure to the seal's barrier cavity, or by an increase in the pressure of the pumped fluid. When this occurs, the pumped fluid is forced between the seal faces. The shallow groove features of typical lift-off face seals do not properly create lift with a pressure reversal condition, resulting in face contact. Due to the relatively wide radial width of lift-off seal faces, significant heat generation results. This can lead to wear and damage of the seal faces, which will then prevent the seal from returning to normal operation as a lift-off seal due to damage to the shallow grooves.
The objective of this aspect of the invention is to provide an improved shallow groove pattern for bidirectional operation having an improved face pattern geometry that permits the seal to maintain lift both in the normal and reversed pressure directions for the seal. This allows the seal to always have a controlled lift between the seal faces regardless of pressure direction, and prevents damage from contact. This feature enables the seal to contain and survive pressure reversal conditions with a return to operation as a lift off gas seal after such an event.
Other objects and purposes of the invention, and variations thereof, will be apparent upon reading the following specification and inspecting the accompanying drawings.
Certain terminology will be used in the following description for convenience and reference only, and will not be limiting. For example, the words “upwardly”, “downwardly”, “rightwardly” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” will refer to directions toward and away from, respectively, the geometric center of the arrangement and designated parts thereof. Said terminology will include the words specifically mentioned, derivatives thereof, and words of similar import.